Multi-metal oxide compounds with a two-phase structure

ABSTRACT

Multimetal oxide materials containing molybdenum, vanadium, antimony, one or more of the elements W, Nb, Ta, Cr and Ce and one or more of the elements Cu, Zn, Co, Fe, Cd, Mn, Mg, Ca, Sr and Ba and having a 2-component structure are used for the gas-phase catalytic oxidative preparation of acrylic acid.

CONTINUING APPLICATION DATA

This application is a 371 of PCT/EP99/02083 filed on Mar. 26, 1999.

SUMMARY OF THE INVENTION

The present invention relates to multimetal oxide materials of theformula I

(A)_(p)(B)_(q)  (I),

where:

A is Mo₁₂V_(a)X¹ _(b)X² _(c)X³ _(d)X⁴ _(e)X⁵ _(f)X⁶ _(g)O_(x),

B is X⁷ ₁Sb_(h)H_(i)O_(y),

X¹ is W, Nb, Ta, Cr and/or Ce, preferably W, Nb and/or Cr,

X² is Cu, Ni, Co, Fe, Mn and/or Zn, preferably Cu, Ni, Co and/or Fe,

X³ is Sb and/or Bi, preferably Sb,

X⁴ is Li, Na, K, Rb, Cs and/or H, preferably Na and/or K,

X⁵ is Mg, Ca, Sr and/or Ba, preferably Ca, Sr and/or Ba,

X⁶ is Si, Al, Ti and/or Zr, preferably Si, Al and/or Ti,

X⁷ is Cu, Zn, Co, Fe, Cd, Mn, Mg, Ca, Sr and/or Ba, preferably Cu, Ni,Zn, Co and/or Fe, particularly preferably Cu and/or Zn, veryparticularly preferably Cu,

a is from 1 to 8, preferably from 2 to 6,

b is from 0.2 to 5, preferably from 0.5 to 2.5,

c is from 0 to 23, preferably from 0 to 4,

d is from 0 to 50, preferably from 0 to 3,

e is from 0 to 2, preferably from 0 to 0.3,

f is from 0 to 5, preferably from 0 to 2,

g is from 0 to 50, preferably from 0 to 20,

h is from 0.1 to 50, preferably from 0.2 to 20, particularly preferablyfrom 0.2 to 5,

i is from 0 to 50, preferably from 0 to 20, particularly preferably from0 to 12,

x and y are each numbers which are determined by the valency andfrequency of the elements in (I) other than oxygen and

p and q are each numbers which differ from zero and the ratio p/q isfrom 20:1 to 1:80, preferably from 10:1 to 1:35, particularly preferablyfrom 2:1 to 1:3,

which contain the moiety (A)_(p) in the form of three-dimensionalregions A of the chemical composition

A: Mo₁₂V_(a)X¹ _(b)X² _(c)X³ _(d)X⁴ _(e)X⁵ _(f)X⁶ _(g)O_(x)

and the moiety (B)_(q) in the form of three-dimensional regions B of thechemical composition

B: X⁷ ₁Sb_(h)H_(i)O_(y)

where the regions A, B are distributed relative to one another as in amixture of finely divided A and finely divided B, with the proviso thatthe multimetal oxide materials (I) are prepared using at least oneseparately preformed oxometallate B,

X⁷ ₁Sb_(h)H_(i)O_(y),

 which is obtainable by preparing a dry blend from suitable sources ofthe elemental constituents of the oxometallate B which contain at leasta part of the antimony in the oxidation state +5 and calcining said dryblend at from 200 to 1200° C., preferably from 200 to 850° C.,particularly preferably from 250 to <600° C., frequently ≦550° C.

The present invention also relates to processes for the preparation ofmultimetal oxide materials (I) and their use as catalysts for thegas-phase catalytic oxidation of acrolein to acrylic acid.

DESCRIPTION OF THE BACKGROUND

WO 96/27437 relates to multimetal oxide materials which contain theelements Mo, V, Cu and Sb as essential components and whose X-raydiffraction pattern has the line of strongest intensity at a 2θ value of22.2°. WO 96/27437 recommends these multimetal oxide materials assuitable catalysts for the gas-phase catalytic oxidation of acrolein toacrylic acid. Furthermore, WO 96/27437 recommends using Sb₂O₃ as anantimony source for the preparation of these multimetal oxide materials.Preparation of an antimony-containing component beforehand is notdescribed in WO 96/27437.

EP-B 235760 relates to a process for the preparation of Sb, Mo, V and/orNb-containing multimetal oxide materials which are suitable as catalystsfor the gas-phase catalytic oxidation of acrolein to acrylic acid. EP-B235760 recommends using an antimony prepared beforehand as an antimonysource for the preparation of these multimetal oxide materials.

The disadvantage of the multimetal oxide materials of the prior art isthat their activity and the selectivity of the acrylic acid formationare not completely satisfactory when they are used as catalysts for thegas-phase catalytic oxidation of acrolein to acrylic acid.

OBJECTS OF THE INVENTION

It is an object of the present invention to provide novel multimetaloxide materials which, when used as catalysts for the gas-phasecatalytic oxidation of acrolein to acrylic acid, have the disadvantagesof the catalysts of the prior art to a reduced extent, if at all.

We have found that this object is achieved by the multimetal oxidematerials (I) defined at the outset.

Very particularly preferred materials (I) are those whose regions A havea composition of the following formula (II)

Mo₁₂V_(a),X¹ _(b),X² _(c),X⁵ _(f),X⁶ _(g),O_(x),  (II),

where

X¹ is W and/or Nb,

X² is Cu and/or Ni,

X⁵ is Ca and/or Sr,

X⁶ is Si and/or Al,

a′ is 2 to 6,

b′ is 0.5 to 2.5,

c′ is 0 to 4,

f′ is 0 to 2,

g′ is 0 to 2 and

x′ is a number which is determined by the valency and frequency of theelements in (II) other than oxygen.

DETAILED DESCRIPTION OF THE INVENTION

It is also advantageous if at least one of the moieties (A)_(p), (B)_(q)of the novel multimetal oxide materials (I) is contained in the latterin the form of three-dimensional regions having the chemical compositionA or B, whose maximum diameters d_(A) and d_(B) (longest connecting linebetween two points present on the surface (interface) of the region andpassing through the center of gravity of the region), respectively, arefrom >0 to 300 μm, preferably from 0.01 to 100 μm, particularlypreferably from 0.05 to 50 μm, very particularly preferably from 0.05 to20 μm.

However, the maximum diameters can of course also be from 0.05 to 1.0 μmor from 75 to 125 μm (the experimental determination of the maximumdiameter is permitted, for example, by a microstructure analysis bymeans of a scanning electron microscope (SEM)).

As a rule, the moiety (B)_(q) is present in the novel multimetal oxidematerials essentially in crystalline form, i.e. as a rule the regions Bessentially comprise small crystallites whose maximum dimension istypically from 0.05 to 20 μm. However, the moiety (B)_(q) can of coursealso be amorphous and/or crystalline.

Particularly preferred novel multimetal oxide materials are those whoseregions B essentially comprise crystallites which have the trirutilestructure type of α- and/or β-copper antimony CuSb₂O₆. α-CuSb₂O₆crystallizes in a tetragonal trirutile structure (E.-O. Giere et al., J.Solid State Chem. 131 (1997), 263-274), whereas β-CuSb₂O₆ has amonoclinically distorted trirutile structure (A. Nakua et al., J. SolidState Chem. 91 (1991), 105-112, or reference diffraction pattern inindex card 17-284 in the JCPDS-ICDD index 1989). In addition, regions Bwhich have the pyrochlore structure of the mineral partzite, a copperantimony oxide hydroxide with the variable composition Cu_(y)Sb_(2-x)(O,OH, H₂O)₆₋₇(y≦2.0≦x≦1), are preferred (B. Mason et al., Mineral. Mag. 30(1953), 100-112, or reference pattern in index card 7-303 of theJCPDS-ICDD index 1996).

Furthermore, the regions B may consist of crystallites which have thestructure of copper antimony Cu₉Sb₄O₁₉ (S. Shimada et al., Chem. Lett.(1983) 1875-1876 or S. Shimada et al., Thermochim. Acta 133 (1988),73-77 or reference pattern in index card 45-54 of the JCPDS-ICDD index)and/or the structure of Cu₄SbO_(4.5) (S. Shimada et al., Thermochim.Acta 56 (1982), 73-82 or S. Shimada et al., Thermochim. Acta 133 (1988),73-77, or reference pattern in index card 36-1106 of the JCPDS-ICDDindex).

Of course, the regions B may also consist of crystallites whichconstitute a mixture of the abovementioned structures.

The novel materials (I) are obtainable in a simple manner, for exampleby first separately preforming oxometallates B,

X⁷ ₁Sb_(h)H_(i)O_(y),

in finely divided form as starting material 1. The oxometallates B canbe prepared by preparing a preferably intimate, advantageously finelydivided dry blend from suitable sources or their elemental constituentsand calcining said dry blend at from 200 to 1200° C., preferably from200 to 850° C., particularly preferably from 250 to <600° C., frequently≦550° C. (as a rule for from 10 min to several hours). All that isessential to the invention is that at least a part of the oxometallatesB of the starting material 1 (referred to below as oxometallates B*) isobtainable by preparing a preferably intimate, advantageously finelydivided dry blend from suitable sources of the elemental constituents ofthe oxometallate B which contain at least a part of the antimony inoxidation state +5 and calcining said dry blend at from 200 to 1200° C.,preferably from 200 to 850° C., particularly preferably from 250 to<600° C., frequently ≦550° C. (as a rule for from 10 min to severalhours). The calcination of the precursors of the oxo-metallates B cangenerally also be carried out under inert gas, but also under a mixtureof inert gas and oxygen, such as air, or under pure oxygen. Calcinationunder a reducing atmosphere is also possible. As a rule, the requiredcalcination time decreases with increasing calcination temperature.Advantageously, the proportion of the oxometallates B* in the finelydivided starting material 1 is at least 10, better at least 20,frequently at least 30 or at least 40, preferably at least 50, evenbetter at least 60, particularly preferably at least 70 or at least 80,frequently at least 90 or 95, very particularly preferably 100, % byweight, based on the starting material 1.

Oxometallates B* are obtainable, for example, by the preparation methodsdescribed in detail in DE-A 24 076 77. Preferred among these is theprocedure in which antimony trioxide and/or Sb₂O₄ are oxidized in anaqueous medium by means of hydrogen peroxide in an amount which is equalto or greater than the stoichiometric amount at from 40 to 100° C. togive antimony(V) oxide hydroxide, aqueous solutions and/or suspensionsof suitable starting compounds of the other elemental constituents ofthe oxometallate B* are added just before this oxidation, during thisoxidation and/or after this oxidation, the resulting aqueous mixture isthen dried (preferably spray-dried (inlet temperature: from 250 to 600°C., outlet temperature: from 80 to 130° C.)) and the intimate dry blendis then calcined as described.

In the process just described, for example, aqueous hydrogen peroxidesolutions having an H₂O₂ content from 5 to 33 or more % by weight may beused. Subsequent addition of suitable starting compounds of the otherelemental constituents of the oxometallate B* is recommended inparticular when these are capable of catalytically decomposing thehydrogen peroxide. However, it would of course also be possible toisolate the resulting antimony(V) oxide hydroxide from the aqueousmedium and to intimately dry-blend it, for example, with suitable finelydivided starting compounds of the other elemental constituents of theoxometallate B* and then to calcine this intimate mixture as described.

It is important that the elemental sources of the oxometallates B, B*are either already oxides or are compounds which can be converted intooxides by heating, in the presence or absence of oxygen.

In addition to the oxides, particularly suitable starting compounds aretherefore halides, nitrates, formates, oxalates, acetates, carbonatesand/or hydroxides (compounds such as NH₄OH, NH₄CHO₂, CH₃COOH, NH₄CH₃CO₂or ammonium oxalate, which disintegrate and/or can be decomposed at thelatest during calcination to give compounds which escape completely ingaseous form, may additionally be incorporated). For the preparation ofoxometallates B, the intimate mixing of the starting compounds cangenerally be carried out in dry or in wet form. If it is effected in dryform, the starting compounds are advantageously used in the form offinely divided powders. However, the intimate mixing is preferablyeffected in wet form. Usually, the starting compounds are mixed with oneanother in the form of an aqueous solution and/or suspension. After theend of the mixing process, the fluid material is dried and is calcinedafter drying. The drying is preferably carried out by spray-drying.

After calcination is complete, the oxometallates B, B* can be comminutedagain (for example by wet or dry milling, for example in a ball mill orby jet-milling) and the particle class, having a maximum particlediameter (as a rule from >0 to 300 μm, usually from 0.01 to 200 μm,preferably from 0.01 to 100 μm, very particularly preferably from 0.05to 20 μm) in the maximum diameter range desired for the novel multimetaloxide (I) can be separated off from the resulting powder, frequentlyessentially comprising spherical particles, by classification to becarried out in a manner known per se (for example wet or dry sieving).

A preferred method of preparation of oxometallates B* of the formula(Cu,Zn)₁Sb_(h)H_(i)O_(y) comprises converting antimony trioxide and/orSb₂O₄ in an aqueous medium by means of hydrogen peroxide initially intoa preferably finely divided Sb(V) compound, for example Sb(V) oxidehydroxide hydrate, adding an ammoniacal aqueous solution of zinccarbonate and/or copper carbonate (which may have, for example, thecomposition Cu₂(OH)₂CO₃) to the resulting aqueous suspension, drying theresulting aqueous mixture, for example spray-drying it in the mannerdescribed, and calcining the resulting powder in the manner described,if necessary after subsequent kneading with water followed by extrusionand drying.

In the case of oxometallates B differing from oxometallates B*, itproves particularly advantageous to start from an aqueous antimonytrioxide suspension and to dissolve therein the X⁷ elements as nitrateand/or acetate, to spray-dry the resulting aqueous mixture in the mannerdescribed and then to calcine the resulting powder in the mannerdescribed.

For the preparation of multimetal oxide materials (I), the startingmaterials 1 preformed as described can then be brought into intimatecontact with suitable sources of the elemental constituents of themultimetal oxide material A

Mo₁₂V_(a)X¹ _(b)X² _(c)X³ _(d)X⁴ _(e)X⁵ _(f)X⁶ _(g)O_(x),

in the desired ratio and a dry blend resulting therefrom can be calcinedat from 250 to 500° C., it being possible to carry out the calcinationunder inert gas (e.g. N₂), a mixture of inert gas and oxygen (e.g. air),reducing gases such as hydrocarbons (e.g. methane), aldehydes (e.g.acrolein) or ammonia, or under a mixture of O₂ and reducing gases (e.g.all of the abovementioned ones), as described, for example, in DE-A 43359 73. In the case of calcination under reducing conditions, it shouldbe ensured that the metallic consituents are not reduced right down tothe element. The calcination time is as a rule a few hours and usuallydecreases with increasing calcination temperature. As is generallyknown, all that is important with regard to the sources of the elementalconstituents of the multimetal oxide material A is that either they arealready oxides or they are compounds which can be converted into oxidesby heating, at least in the presence of oxygen. In addition to theoxides, particularly suitable starting compounds are halides, nitrates,formates, oxalates, citrates, acetates, carbonates or hydroxides.Suitable starting compounds of Mo, V, W and Nb are also their oxocompounds (molybdates, vanadates, tungstates and niobates) or the acidsderived from these.

The starting material 1 can be brought into intimate contact with thesources of the multimetal oxide material A (starting material 2) eitherin dry or in wet form. In the latter case, it is merely necessary toensure that the preformed multimetal oxides B, B* do not go intosolution. In an aqueous medium, the latter is usually ensured at a pHwhich does not differ too greatly from 7 and at ≦60° C. and ≦40° C.,respectively. If said substances are brought into intimate contact inwet form, drying is then usually carried out to give a dry material(preferably by spray-drying). Such a dry material is automaticallyobtained in dry blending.

Possible mixing methods are thus, for example:

a. mixing a dry, finely divided, preformed starting material 1 with dry,finely divided starting compounds of the elemental constituents of thedesired multimetal oxide A in the desired ratio in a mixer, kneader ormill;

b. preforming a finely divided multimetal oxide A by intimate mixing ofsuitable starting compounds of its elemental constituents (dry or wet)and then calcining the resulting intimate dry blend at from 250 to 500°C. (regarding the calcination time, calcination atmosphere and elementsources, statements made above are applicable); converting the preformedmultimetal oxide A into finely divided form and mixing it with thefinely divided starting material 1 in the desired ratio as in a.; inthis mixing method, a final calcination of the resulting mixture is notessential;

c. stirring the required amount of preformed starting material 1 into anaqueous solution and/or suspension of starting compounds of theelemental constituents of the desired multimetal oxide A and thenspray-drying the mixture; instead of the starting compounds of theelemental constituents of the desired multimetal oxide A, it is ofcourse also possible to use a multimetal oxide A itself, alreadypreformed according to b.

All mixing methods between a., b. and/or c. can of course also be used.The resulting intimate dry blend can then be calcined in the mannerdescribed and then shaped to give the desired catalyst geometry, or viceversa. In principle, the calcined dry blend (or possibly uncalcined dryblend where mixing method b. is used) can however also be used in theform of a powder catalyst.

Our own investigations have shown that, on calcination of the dry blendcomprising the starting material 1 and the starting material 2,essentially no fusion of the components of the starting material 1 withthose of the starting material 2 takes place and the structure type ofthe crystallites contained in the starting material 1 are oftenessentially retained as such.

As indicated above, this opens up the possibility, after milling of thepreformed starting mixture 1, to separate off the particle class havingthe maximum particle diameter (as a rule from >0 to 300 μm, preferablyfrom 0.01 to 100 μm, particularly preferably from 0.05 to 20 μm) in themaximum diameter range desired for the multimetal oxide material (I)from the resulting powder, frequently comprising essentially sphericalparticles, by a classification to be carried out in a manner known perse (for example wet or dry sieving) and thus to use said particle classin a tailor-made manner for the preparation of the desired multimetaloxide material.

When the novel multimetal oxide materials (I) are used as catalysts forthe gas-phase catalytic oxidation of acrolein to acrylic acid, theshaping to give the desired catalyst geometry is preferably carried outby application to preformed inert catalyst carriers, it being possibleto effect the application before or after the final calcination. Theconventional carrier materials, such as porous or nonporous aluminas,silica, thorium dioxide, zirconium dioxide, silicon carbide or silicatessuch as magnesium silicate or aluminum silicate, may be used. Thesupports may be regularly or irregularly shaped, regularly shapedsupports having pronounced surface roughness, for example spheres orhollow cylinders, being preferred. Among these, in turn, spheres areparticularly advantageous. It is particularly advantageous to useessentially nonporous spherical steatite carriers with a rough surface,whose diameter is from 1 to 8 mm, preferably from 4 to 5 mm. The layerthickness of the active material is advantageously chosen in the rangefrom 50 to 500 μm, preferably from 150 to 250 μm. It should be pointedout here that, for coating the supports, in the preparation of suchcoated catalysts, the powder material to be applied is as a rulemoistened and, after application, is dried, for example by means of hotair.

For the preparation of the coated catalysts, the coating of the supportsis as a rule carried out in a suitable rotatable container, aspreviously disclosed, for example, in DE-A 2909671 or in EP-A 293859. Asa rule, the relevant material is calcined before coating the carrier.

The coating and calcination process according to EP-A 293 859 can beused in a suitable manner known per se so that the resulting multimetaloxide active materials have a specific surface area of from 0.50 to 150m²/g, a specific pore volume of from 0.10 to 0.90 cm³/g and a porediameter distribution such that at least 10% of the total pore volume isassociated with each of the diameter ranges from 0.1 to <1 μm, from 1.0to <10 μm and from 10 μm to 100 μm. Moreover, the pore diameterdistributions stated as being preferred in EP-A 293 859 may beestablished.

The novel multimetal oxide materials can of course also be operated asunsupported catalysts. In this respect, the intimate dry blendcomprising the starting materials 1 and 2 is preferably compacteddirectly to give the desired catalyst geometry (for example by means ofpelleting or extrusion), it being possible, if necessary, to addconventional assistants, for example graphite or stearic acid aslubricants, and/or molding assistants and reinforcing agents, such asmicrofibers of glass, asbestos, silicon carbide or potassium titanate,and calcined. Here, too, calcination can in general be effected prior toshaping. Preferred geometries for unsupported catalysts are hollowcylinders having an external diameter and a length of from 2 to 10 mmand a wall thickness of from 1 to 3 mm.

The novel multimetal oxide materials are particularly suitable ascatalysts having high activity and selectivity (at given conversion) forthe gas-phase catalytic oxidation of acrolein to acrylic acid. Acroleinproduced by the catalytic gas-phase oxidation of propene is usually usedin the process. As a rule, the acrolein-containing reaction gases fromthis propene oxidation are used without intermediate purification.Usually, the gas-phase catalytic oxidation of acrolein is carried out intube-bundle reactors as a heterogeneous fixed-bed oxidation. Oxygen,advantageously diluted with inert gases (for example in the form ofair), is used as an oxidizing agent in a manner known per se. Suitablediluent gases are, for example, N₂, CO₂, hydrocarbon, recycled reactionexit gases and/or steam. As a rule, an acrolein:oxygen:steam:inert gasvolume ratio of 1:(1 to 3):(0 to 20):(3 to 30), preferably 1:(1 to3):(0,5 to 10):(7 to 18), is established in the acrolein oxidation. Thereaction pressure is in general from 1 to 3 bar and the total spacevelocity is preferably from 1000 to 3500 l(S.T.P.)/l/h. Typicalmultitube fixed-bed reactors are described, for example, in DE-A2830765, DE-A 2 201 528 or U.S. Pat. No. 3,147,084. The reactiontemperature is usually chosen so that the acrolein conversion in asingle pass is above 90%, preferably above 98%. Usually, reactiontemperatures of from 230 to 330° C. are required in this respect.

In addition to the gas-phase catalytic oxidation of acrolein to acrylicacid, the novel products are however also capable of catalyzing thegas-phase catalytic oxidation of other organic compounds, in particularother alkanes, alkanols, alkanals, alkenes and alkenols, preferably with3 to 6 carbon atoms (e.g. propylene, methacrolein, tert-butanol, themethyl ether of tert-butanol, isobutene, isobutane or isobutyraldehyde),to olefinically unsaturated aldehydes and/or carboxylic acids, and thecorresponding nitriles (ammoxidation, especially of propene toacrylonitrile and of isobutene or tert-butanol to methacrylonitrile).The preparation of acrolein, methacrolein and methacrylic acid may bementioned by way of example. However, they are also suitable for theoxidative dehydrogenation of olefinic compounds.

Unless stated otherwise, the conversion, selectivity and residence timeare defined in this publication as follows: ${\begin{matrix}{{Conversion}\quad C\quad {of}} \\{{acrolein}\quad (\%)}\end{matrix} = {\frac{{{no}.\quad {of}}\quad {moles}\quad {of}\quad {acrolein}\quad {converted}}{{{no}.\quad {of}}\quad {moles}\quad {of}\quad {acrolein}\quad {used}} \times 100}};$${\begin{matrix}{{Selectivity}\quad S\quad {of}\quad {the}} \\{{acrylic}\quad {acid}\quad {formation}\quad \%}\end{matrix} = {\frac{\begin{matrix}{{{no}.\quad {of}}\quad {moles}\quad {of}\quad {acrolein}} \\{{converted}\quad {into}\quad {acrylic}\quad {acid}}\end{matrix}}{\begin{matrix}{{total}\quad {{no}.\quad {of}}\quad {moles}\quad {of}} \\{{acrolein}\quad {converted}}\end{matrix}} \times 100}};$${{Residence}\quad {time}\quad \left( \sec \right)} = {\frac{\begin{matrix}{{empty}\quad {reactor}\quad {volume}\quad {filled}} \\{{with}\quad {catalyst}\quad (l)}\end{matrix}}{\begin{matrix}{{Synthesis}\quad {gas}\quad {throughput}} \\{{l\left( {S.T.P.} \right)}/h}\end{matrix}} \times 3600.}$

EXAMPLES I. Catalyst Preparation Example

a) Preparation of the Starting Material 1

946.0 g of Sb₂O₃ having an Sb content of 83.0% by weight were suspendedin 4 l of water while stirring. 822.4 g of a 30% strength by weightaqueous H₂O₂ solution were added at room temperature (25° C.).Thereafter, the suspension was heated to 100° C. in the course of 1 hourwith further stirring and was refluxed at this temperature for 5 hours.A solution of 595.6 g of Cu(CH₃COO)₂.H₂O having a Cu content of 32.0% byweight in 4 l of water was then added to the aqueous suspension at 100°C. in the course of 30 minutes, the temperature of the total aqueousmixture decreasing to 60° C. At this temperature, 407.9 g of a 25%strength by weight aqueous ammonia solution were then added. Thereafter,the aqueous suspension was stirred for a further 2 hours at 80° C. andthen cooled to room temperature (25° C.). Finally, the aqueoussuspension was spray-dried (inlet temperature: 350° C., outlettemperature: 110° C.). The resulting spray-dried powder was heatedstepwise in a rotary oven (2 l internal volume) with the passage of 100l(S.T.P.)/h of air, initially to 150° C. in the course of 1 hour, thento 200° C. in the course of 4 hours and finally to 300° C. in the courseof 2 hours, and was kept at the last-mentioned temperature for 1 hour.Thereafter, the powder obtained was heated to 400° C. in the course of1.5 hours and thermostatted at this temperature for 1 hour. The powderobtained had a specific BET surface area (determined according to DIN66131, by gas adsorption (N₂) according to Brunauer-Emmet-Teller) of48.5 m²/g and the stoichiometry CuSb_(2.15)O_(y) (y≦6.375). The powderexhibited the X-ray diffraction reflections of the mineral partzite andthus corresponded to reference spectrum 7-0303 of the JCPDS-ICDD index1996.

b) Preparation of the Starting Material 2

682.4 g of ammonium heptamolybdate tetrahydrate (81.5% by weight ofMoO₃), 131.0 g of ammonium metavanadate (77.3% by weight of V₂O₅) and114.6 g of ammonium paratungstate heptahydrate (89.0% by weight of WO₃)were dissolved in succession in 5030 g of water at 95° C. The aqueoussolution (starting material 2) was thus based on the followingstoichiometry:

 Mo_(3.86)V_(1.11)W_(0.44){circumflex over(═)}(MO₁₂V_(3.45)W_(1.37))_(0.32).

c) Preparation of a Multimetal Oxide Material M and of a Coated CatalystCC

The clear, orange-colored solution obtained above (starting material 2)was cooled to 25° C. and 150.0 g of ammonium acetate were added. 239.5 gof the starting material 1 were stirred into the aqueous solution cooledto 25° C. so that the molar ratio of the abovementioned stoichiometricunits was 0.56 (starting material 1) to 0.32 (starting material 2). Theresulting suspension was stirred for a further 1 hour at 25° C. and theaqueous mixture was then spray-dried. The spray-dried powder was thenkneaded with a mixture of 70% by weight of water and 30% by weight ofacetic acid (0.35 kg of liquid/kg of spray-dried powder) (LUK 2.5kneader from Werner und Pfleiderer). The kneaded material obtained wasdried for 16 hours at 110° C. in a through-circulation oven throughwhich air flowed. The subsequently comminuted kneaded material wascalcined in a cylindrical rotary oven (internal diameter: 12.5 cm,heated length: 50 cm) through which an air/nitrogen mixture (15l(S.T.P.)/h of air and 200 l(S.T.P.)/h of nitrogen) flowed. 700 g ofmaterial to be calcined were introduced into the heated zone of therotary oven. In the calcination, heating was initially carried out to325° C. in the course of 60 minutes and this temperature was thenmaintained for 4 hours. Thereafter, heating was carried out to 400° C.in the course of 20 minutes and this temperature was maintained for 1hour. The resulting catalytically active multimetal oxide material hadthe following gross stoichiometry:

Mo_(3.86)V_(1.11)W_(0.44)Cu_(0.56)Sb_(1.20)O_(x)≡(Mo₁₂V_(3.45)W_(1.37))_(0.32)(CuSb_(2.15)O_(y))_(0.56).

After the calcined active material had been milled, it was used to coatnonporous steatite spheres having a rough surface and a diameter of from4 to 5 mm in a rotating drum, in an amount of 60 g of active powder per400 g of steatite spheres, with simultaneous addition of water (coatingprocess according to DE-A 4 442 346). The coated catalyst CC obtainedwas then dried with air at 110° C.

Comparative Example

The preparation of a comparative multimetal oxide material CM and of acomparative coated catalyst CCC was carried out as in the example,except that no hydrogen peroxide was used for the preparation of thestarting material 1.

II. Use of the Coated Catalysts from I. as Catalysts for the Gas-phaseOxidation of Acrolein to Acrylic Acid

The coated catalysts were introduced into a tubular reactor (V2Astainless steel, 25 mm internal diameter, 2000 g catalyst bed, heated bymeans of a salt bath) and, with the use of a residence time of 2.0seconds, were loaded with a gaseous mixture having the composition

5% by volume of acrolein,

7% by volume of oxygen,

10% by volume of steam and

78% by volume of nitrogen.

The salt bath temperature was always adjusted so that, after forming wascomplete, a standard acrolein conversion C of 99% resulted after asingle pass. The product gas mixture flowing from the reactor wasanalyzed by gas chromatography. The results of the selectivity of theacrylic acid formation using the various catalysts and the required saltbath temperatures are shown in the table below:

Salt bath temperature Catalyst (° C.) S% CC 267 95.5 CCC 272 93.8

We claim:
 1. A multimetal oxide material of the formula I(A)_(p)(B)_(q)  (I), where: A is Mo₁₂V_(a)X¹ _(b)X² _(c)X³ _(d)X⁴ _(e)X⁵_(f)X⁶ _(g)O_(x) B is X⁷ _(l)Sb_(h)H_(i)O_(y), X¹ is W, Nb, Ta, Crand/or Ce, x² is Cu, Ni, Co, Fe, Mn and/or Zn, x³ is Sb and/or Bi, x⁴ isLi, Na, K, Rb, Cs and/or H, x⁵ is Mg, Ca, Sr and/or Ba, x⁶ is Si, Al, Tiand/or Zr, X⁷ is Cu, Zn, Co, Fe, Cd, Mn, Mg, Ca, Sr and/or Ba, a is from1 to 8, b is from 0.2 to 5, c is from 0 to 23, d is from 0 to 50, e isfrom 0 to 2, f is from 0 to 5, g is from 0 to 50, h is from 0.1 to 50, iis from 0 to 50, x and y are each numbers which are determined by thevalency and frequency of the elements in (I) other than oxygen and p andq are each numbers which differ from zero and the ratio p/q is from 20:1to 1:80, which contains the moiety (A)_(p) in the form ofthree-dimensional regions A of the chemical composition A: Mo₁₂V_(a)X¹_(b)X² _(c)X³ _(d)X⁴ _(e)X⁵ _(f)X⁶ _(g)O_(x) and the moiety (B)_(q) inthe form of three-dimensional regions B of the chemical composition B:X⁷ _(l)Sb_(h)H_(i)O_(y) where the regions A, B are distributed relativeto one another as in a mixture of finely divided A and finely divided B,with the proviso that the multimetal oxide materials (I) are preparedusing at least one separately preformed oxometallate B, X⁷_(l)Sb_(h)H_(i)O_(y),  which is obtained by preparing a dry blend fromsuitable sources of the elemental constituents of the oxometallate Bwhich contain at least a part of the antimony in the oxidation state +5and calcining said dry blend at from 200 to 1200° C.
 2. A process forthe preparation of a multimetal oxide as claimed in claim 1, in which anoxometallate B X⁷ _(l)Sb_(h)H_(i)O_(y), is preformed in finely dividedform and then processed with sources of the elemental constituents ofthe multimetal oxide material A Mo₁₂V_(a)X¹ _(b)X² _(c)X³ _(d)X⁴ _(e)X⁵_(f)X⁶ _(g)O_(x), in the desired ratio to give a dry blend and thelatter is calcined at from 250 to 500° C., wherein at least a portion ofthe oxometallate B is obtained by preparing a dry blend from sources ofthe elemental constituents of the oxometallate B which contain at leasta part of the antimony in oxidation state +5 and calcining said dryblend at from 200 to 1200° C.
 3. A process for the preparation of anoxometallate B of the formula X⁷ _(l)Sb_(h)H_(i)O_(y), where: X⁷ is Cuand/or Zn, h is from 0.1 to 50, i is from 0 to 50 and y is a numberwhich is determined by the valency and frequency of the elements in theformula other than oxygen, wherein antimony trioxide or Sb₂O₄ is firstoxidized in an aqueous medium by means of hydrogen peroxide to an Sb(V)compound, an ammoniacal aqueous solution of zinc carbonate or coppercarbonate is added to the resulting aqueous suspension and the mixtureobtained is dried and is calcined at from 20 to 1200° C.
 4. A processfor the gas-phase catalytic oxidative preparation of acrylic acid fromacrolein, comprising reacting acrylic acid in the presence of acatalytically effective amount of a multimetal oxide as claimed in claim1 in a gas phase.
 5. A process as claimed in claim 4, wherein reactionis conducted at a pressure of 1 to 3 bar, the total space velocity isfrom 1000 to 3500 l (S.T.P.)/l/h, the temperature is from 230 to 330°C., the acrolein:oxygen:steam:inert gas volume ratio is 1:(1 to 3):(0 to20):(3 to 30), and the acrolein conversion in a single pass is above90%.
 6. The multimetal oxide material as claimed in claim 1, wherein atleast one of (A)_(p) and (B)_(q) is in the form of three-dimensionalregions having the chemical composition A or B, whose maximum diametersd_(A) and d_(B), respectively, are from >0 to 300 μm.
 7. The multimetaloxide material as claimed in claim 6, wherein the maximum diameters arefrom 0.01 to 100 μm.
 8. The multimetal oxide material as claimed inclaim 6, wherein the maximum diameters are from 0.05 to 50 μm.
 9. Themultimetal oxide material as claimed in claim 6, wherein the maximumdiameters are from 0.05 to 20 μm.
 10. The multimetal oxide material asclaimed in claim 6, wherein the maximum diameters are from 0.05 to 1.0μm.
 11. The multimetal oxide material as claimed in claim 6, wherein themaximum diameters are from 75 to 125 μm.
 12. The multimetal oxidematerial as claimed in claim 1, wherein regions B comprise crystalliteswhich have the trirutile structure of α- and β-copper antimony CuSb₂O₆.13. The multimetal oxide material as claimed in claim 1, wherein regionsB have the pyroclore structure of the mineral partzite.
 14. Themultimetal oxide material as claimed in claim 1, wherein regions Bconsist of crystallites which have the structure of copper antimonyCu₉Sb₄O₁₉.